Articles

What is a Chronometer rating on a watch?

Simply put, a Chronometer is a rating for a watch in regards to its accuracy. However a Chronometer rating not only applies to wrist watches; it can also apply to pocket watches, marine and observatory chronometers and various other scientific timing equipment.

The history of the Chronometer rating

For a wrist watch to be certified as a â€śChronometerâ€ť, it must first go the non-profit company in Switzerland called the COSC which stands for ContrĂ´le Officiel Suisse des ChronomĂ¨tres.
The history of the COSC goes back to the end of the 19th Century, and was founded by five watchmaking states of Switzerland: Geneva, Bern, NeuchĂ˘tel, Vaud and Solothurn, together with the Federation of the Swiss Watch Industry (FHS).
Originally, a chronometer was used as a timekeeper that was precise enough to be used as a portable time standard, usually in order to determine longitude by means of celestial navigation
Today there are 3 laboratories which now test the movements submitted by watch companies to be certified as a chronometer, which are in Geneva, Bienne and Le Locle.

What is involved in rating a watch as a Chronometer?

A watch movement isnâ€™t simply submitted to the COSC for testing, nor is one watch movement representing multiple movements from the one watch company.
Each watch movement is individually tested, and if the watch movement passes the strict criteria defined by the COSC, it is then given an individual officially recorded serial number and certificate.
If the movement is mechanical, i.e. not a quartz powered movement, the movement is then tested over 15 days, in 5 different positions, and at 3 different temperatures, and during this time the movement must not gain more than 6 seconds, and not lose more than 4 seconds per day.

The following graph shows the different temperature and positions that the movement is placed in.
Day Temperature Position

Day

0

2

3

4

5

6

7

8

9

10

11

12

13

14

15

Temp.

23Â°C

23Â°C

23Â°C

23Â°C

23Â°C

23Â°C

23Â°C

23Â°C

23Â°C

23Â°C

8Â°C

23Â°C

38Â°C

23Â°C

23Â°C

Position

6 oâ€™clock up

6 oâ€™clock up

3 oâ€™clock up

3 oâ€™clock up

9 oâ€™clock up

9â€™clock up

Dial down

Dial down

Dial up

Dial up

Dial up

Dial up

Dial up

6 oâ€™clock up

6 oâ€™clock up

Once the individual watch movement has passed this criteria, the word “Chronometer” can then be written on the dial.

Why are some Swiss watches not Certified?

Each year approximately 1,000,000 watches are sent to the COSC to be certified as a Chronometer. This sounds like a very large number â€“ which in itself it is, however this number represents around 3% of the annual production of watches.
Rolex is widely known as the watch company that supplies the most watches to the COSC to be certified. Each year approximately 83% of the watches certified COSC come from the Rolex factory.
It should be noted that not all of the watch companies send their watches to get certified. Some very well and very highly respected firms such as Jaeger-LeCoultre, IWC, Audemars Piguet, Breguet, Piaget and A. Lange & SĂ¶hne are among the expensive brands that do not submit their movements to the COSC.